1. Field of the Invention
This invention relates generally to apparatus for actuating the intake and/or exhaust valves of an internal combustion engine, and pertains more especially to apparatus employing a specially configured cam unit for providing a relatively quiet valve operation that does not require frequent lash adjustments.
2. Description of the Prior Art
Generally, valves, whether they be intake or exhaust valves, have usually been opened and closed through the agency of rocker arms, there being one for each intake valve and another for each exhaust valve. The rocker arms, when pivoted by the particular cam associated therewith, will cause the rocker arm to bear against the upper end of the valve member, more specificially against a lash cap, to cause the valve to open. This entails the overcoming of a spring action that normally closes the valve but yields when the particular cam for that valve acts against the rocker arm in a manner to force the valve open against the action of the spring. In some cases, there are vertical pushrods extending upwardly from the cams that bear against one end of a rocker arm in each instance. In less encountered situations, the cams bear directly against the lash caps to cause the valves to open.
Irrespective of which design is employed, there is a certain amount of wear that occurs between the cams and the rocker arms, or the cams and the lash caps when the cams act directly on the lash caps. A significant proportion of the wear results from a single cam constantly bearing against the same follower surface durings its entire rotation, whether the follower surface be part of a rocker arm or a lash cap. Such continuous and repetitious engagement of the cam lobe against the follower laterally wears a groove in the follower's working surface. As the groove becomes deeper and deeper, the erosive action, the clearance between the cam's base circle and the bottom of the constantly deepening groove increases; hence, the increase in lash.
It is the progressive increase in lash that requires rather frequent adjustments, usually by adding shims between the lash cap and end of the valve stem. As the amount of lash increases, however, the valve noise increases. Consequently, the solution to the problem of increasing lash has at times been met by using hydraulic valve lifters. Hydraulic valve lifters, however, are more costly, and require heavier springs to seat the valves when hydraulic lifters are employed because the springs must exert a greater amount of closing force in order to overcome the hydraulic pressure on the lifter. When the spring constant or strength is increased, then more power is needed to open the valves.